Begin by preparing your slow cooker. Coat the inner pot of a 5- to 6-quart slow cooker with nonstick spray or line it with a nonstick liner. This ensures the crisp comes out easily once it’s done cooking.
In a large mixing bowl, combine the old-fashioned rolled oats, white whole wheat flour, sliced almonds (if using), ground cinnamon, and kosher salt. Stir the dry ingredients together until they are well combined.
Next, pour the melted butter and maple syrup over the top of the dry mixture. Using a spatula or wooden spoon, gently stir to evenly moisten all the ingredients. You want them to be well combined but not overmixed.
If you're using frozen berries, rinse them to remove any large ice crystals. They don’t need to be completely thawed, just de-iced. Place the berries in another large bowl.
To the berries, add the lemon juice, vanilla extract, and ½ tablespoon of cornstarch (if using fresh berries). If using frozen berries, add 1 tablespoon of cornstarch. Gently stir to combine, ensuring the berries are well coated.
Transfer the berry mixture into the prepared slow cooker, spreading it evenly across the bottom. This layer will bubble and burst with flavor as it cooks.
Evenly sprinkle the oat mixture over the top of the berries. Make sure to cover the berries completely; this will create a delicious crust on top.
To help the topping crisp up nicely, lightly coat the top of the crisp with nonstick spray. This step is essential for achieving that golden crisp topping!
Lay a sheet of paper towel or a kitchen towel over the top of the slow cooker. Allow it to overhang the sides; this will help absorb moisture during cooking and ensure a crispy texture.
Cover the slow cooker with its lid and cook the Slow Cooker Triple Berry Crisp on high for 1 to 2 hours or on low for 2 to 3 hours. Keep an eye on it, as cooking times may vary depending on your slow cooker. You’ll know it’s done when the berries are bubbly and the top is a lovely golden brown.
If you’re using frozen berries, be aware that it may require additional cooking time for everything to heat through and thicken properly. Once done, remove the lid and allow it to cool for a few minutes before serving.
Serve the Slow Cooker Triple Berry Crisp warm, topped with a scoop of vanilla ice cream, Greek yogurt, or a dollop of whipped cream, if desired. Enjoy every bite!